1. Field of the Invention
The invention relates to an optical touch technology, and particularly relates to a touch point sensing method and an optical touch system.
2. Description of Related Art
In the optical touch technology, the light reflective characteristic of the screen frame is utilized. Light is emitted onto the frame and reflected, and the optical sensors at the left and right sides are used to capture the images in an optical detecting region. If there is a touch object in the detecting region, light reflected from the frame is blocked. Therefore, there is a shadow in the image captured by the optical sensor, and the touch position is thus calculated based on the position of the shadow. More specifically, the touch position is determined by an original image waveform when there is no touch object detected in the detecting region and a current image waveform detected by the optical sensor. If the current image waveform and the original image waveform are the same, it is determined that there is no touch object. If there is a difference between the current image waveform and the original image waveform, it is determined that there is a touch object.
When there is only one touch object, each of the two sensors detects one shadow. Based on the position of the shadow, the position of the touch object can be correctly calculated. However, when there are two touch objects, since each of the sensors detects two shadows, and four points are derived based on the two shadows respectively detected by each of the two sensors. Two of the four points are correct touch points (i.e., real points), and the other two points are invalid touch points (i.e., ghost points), and the ghost points need to be eliminated. However, under some circumstances (e.g., the real points and ghost points are overlapped and an extending line of the real and ghost points passes through the optical sensor), the optical sensor may misjudge the ghost point as a real point and thus calculate the touch position incorrectly.